This invention relates to an induction heating apparatus for utilizing electromagnetic induction to heat a workpiece and, more particularly, to an induction heating apparatus of the transverse flux heating type which is used to heat a workpiece transported through the induction heating apparatus by producing magnetic flux crossing the workpiece in a direction substantially perpendicular to the surface of the workpiece. It will be appreciated that the term "workpiece" as used throughout this invention is intended to include metal strips, metal sheets, metal plates, and other conductive thin members.
Induction heating apparatus have been employed in drying, hardening, annealing, preheating, and other heating processes whereby heat is generated within the part to be treated by electromagnetic induction. A rapidly alternating magnetic field is produced in a pair of specially designed heating coil units between which a workpiece is transported at a predetermined speed. The magnetic field induces an electric potential in the workpiece, causing heating because of I.sup.2 R losses. For this purpose, the heating coil units are supplied with a high- or intermediate-frequency alternating current from a suitable power source. High frequency is generally used for shallow heating, while intermediate frequency is used for applications requiring deeper heating.
One example of the heating coil unit used in conventional induction heating apparatus includes a pair of heating coil units disposed in spaced-parallel relation to permit transportation of the workpiece in a direction between the heating coil units, each of the heating coil units including a plurality of spaced-parallel transverse conductors extending in a direction substantially perpendicular to the direction of transportation of the workpiece. The transverse conductors are connected in series and supplied with a high- or intermediate-frequency alternating current to produce a rapidly alternating magnetic field crossing the workpiece in a direction perpendicular to the surface of the workpiece.
One problem associated with such conventional induction heating apparatus is that the heated workpiece has a temperature not uniform over its width. Particularly, the workpiece temperature has troughs at positions somewhat inner from the opposite side edges of the workpiece, causing undesirable effects in the following workpiece processing steps. It is very difficult to provide uniform temperature over the full width of the workpiece even with adjustments of the length of the transverse conductors.